Exhaust gas recirculation in internal combustion engines is currently part of many engine concepts in order to meet prescribed emission limits, most notably in Europe. It serves to reduce nitrogen oxides in the combustion of fuels in gasoline and diesel engines in that a portion of the exhaust gases is deliberately conducted back to the combustion chamber of the internal combustion engine. At the present time, a distinction is made between two methods of exhaust gas recirculation in internal combustion engines, namely between internal exhaust gas recirculation by overlaps in the valve opening times by means of camshaft regulation or variable valve drives on the one hand, and external exhaust gas recirculation by means of a return line between the exhaust and intake trains and a control device located in the return line to control the throughput rate on the other hand.
Such a control device for external exhaust gas recirculation in internal combustion engines is known from DE 10 2006 053,716 A1, for example, and consists essentially of a throttle valve housing with a gas channel and a throttle valve that controls the passage of gas through the gas channel and is connected to a throttle valve shaft that extends transversely through the gas channel and can be pivoted by a positioning device. The throttle valve shaft in this design is supported on both sides of the gas channel in two throttle valve bearings arranged in the throttle valve housing, such as are also previously known from DE 10 2006 053,716 A1. Throttle valve bearings of this type, implemented as needle sleeves, consist primarily of a thin-walled outer ring formed by non-cutting means and of a needle roller and cage assembly that is inserted in this outer ring and is composed of a plurality of bearing needles and a needle cage that carries the bearing needles at uniform spacings in the circumferential direction. In this design, the outer ring has a radially inward facing fixed rim at one of its axial sides that is formed thereon during its fabrication, and has at its other axial side an inward facing flanged rim that is formed thereon after insertion of the needle roller and cage assembly and by means of which the needle roller and cage assembly is retained in the outer ring. In addition, one or more seals are located on both sides between these rims and the needle roller and cage assembly, by means of which the throttle valve bearing is sealed to prevent exhaust gas condensates from flowing through.
In new generations of engines in the Euro 5/US 7 emissions classes, however, it has been shown under long-term operating conditions that with increasing quantities of so-called blow-by gases, the throttle valve bearings of such control devices are exposed to correspondingly increased quantities of exhaust gas condensates that penetrate through the housing bores for the throttle valve shaft to the throttle valve bearings. These exhaust gas condensates have an oily/watery to oily/acidic composition, so that the needle sleeves of the throttle valve bearings, which as a general rule are made of hardened ball and roller bearing steel with low alloying content and low carbon content, are initially subject to increased attack by external corrosion, which also propagates into their interior despite the multiple sealing of the throttle valve bearings. In this process, the condensate first forms a rust layer at the outer rims of the needle sleeves, which spreads to the interior of the needle sleeves via their manufacturing-induced cut edges and advances to the seals of the throttle valve bearings. As the corrosion progresses further, rust creep then takes place in the seals, resulting in damage and the failure of the throttle valve bearings in conjunction with functional failure of the throttle valve.
A first countermeasure, the avoidance of corrosion of the throttle valve bearings by a corrosion-reducing zinc-iron coating of the outer rings, for example using Corrotect®, has heretofore proven unsuitable, since the acid-bearing condensate of the exhaust gases dissolves the zinc-iron coating, and thus its sacrificial anodic action that provides the corrosion protection is lost. Although another countermeasure, proposed in WO 2009/103,378 A1, to produce the needle sleeves of both throttle valve bearings from a high-alloy, corrosion-resistant sheet steel by deep-drawing without further heat and/or surface treatment, has indeed achieved the sought-after success, this measure has proven uneconomical on the whole because of the increased production and tooling costs required by such a material.